LTE, or Long Term Evolution, is a name for research and development involving the Third Generation Partnership Project (3GPP), to identify technologies and capabilities that can improve systems such as the UMTS. The present invention involves the long term evolution (LTE) of 3GPP. Implementations of wireless communication systems, such as UMTS (Universal Mobile Telecommunication System), may include a radio access network (RAN). In UMTS, the RAN is called UTRAN (UMTS Terrestrial RAN). Of interest to the present invention is an aspect of LTE referred to as “evolved UMTS Terrestrial Radio Access Network,” or E-UTRAN.
In general, in E-UTRAN resources are assigned more or less temporarily by the network to one or more user equipment terminals (UE) by use of allocation tables, or more generally by use of a downlink resource assignment channel. Users are generally scheduled on a shared channel every transmission time interval (TTI) by a Node B or an evolved Node B (e-Node B). A current working assumption for LTE is that users are explicitly scheduled on a shared channel every transmission time interval (TTI) by an eNodeB. An eNodeB is an evolved Node B and is the UMTS LTE counterpart to the term “base station” in the Global System for Mobile Communication (GSM). In order to facilitate the scheduling on the shared channel, the e-Node B transmits an allocation in a downlink control channel to the UE. The allocation information may be related to both uplink and downlink channels. The allocation information may include information about which resource blocks in the frequency domain are allocated to the scheduled user(s), which modulation and coding schemes to use, what the transport block size is, and the like.
An example of the E-UTRAN architecture is illustrated in FIG. 1. This example of E-UTRAN consists of eNodeBs, providing the E-UTRA user plane (RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the UE. The eNodeBs are interconnected with each other by means of the X2 interface. The eNodeBs are also connected by means of the S1 interface to the EPC (evolved packet core) more specifically to the MME (mobility management entity) and the UPE (user plane entity). The S1 interface supports a many-to-many relation between MMEs/UPEs and eNodeBs. The S1 interface supports a functional split between the MME and the UPE. The MMU/UPE in the example of FIG. 1 is one option for the access gateway (aGW).
In the example of FIG. 1, there exists an X2 interface between the eNodeBs that need to communicate with each other. For exceptional cases (e.g. inter-PLMN handover), LTE_ACTIVE inter-eNodeB mobility is supported by means of MME/UPE relocation via the S1 interface.
The eNodeB may host functions such as radio resource management (radio bearer control, radio admission control, connection mobility control, dynamic allocation of resources to UEs in both uplink and downlink), selection of a mobility management entity (MME) at UE attachment, routing of user plane data towards the user plane entity (UPE), scheduling and transmission of paging messages (originated from the MME), scheduling and transmission of broadcast information (originated from the MME or O&M), and measurement and measurement reporting configuration for mobility and scheduling. The MME/UPE may host functions such as the following: distribution of paging messages to the eNodeBs, security control, IP header compression and encryption of user data streams; termination of U-plane packets for paging reasons; switching of U-plane for support of UE mobility, idle state mobility control, SAE bearer control, and ciphering and integrity protection of NAS signaling. The invention is related to LTE, although the solution of the present invention may also be applicable to present and future systems other than LTE.
In general, E-UTRAN may use orthogonal frequency division multiplexing (OFDM) as the multiplexing technique for a downlink connection between the eNode B and the UE terminal, in which different system bandwidths from 1.25 MHz to 20 MHz are applied. Using OFDM may allow for link adaptation and user multiplexing in the frequency domain. However, to utilize the potential of multiplexing in the frequency domain the Node B or eNode B needs to have information related to the instantaneous channel quality. In order for the Node B or eNode B to be informed of the channel quality, the user equipment terminal provides channel quality indicator (CQI) reports to the eNode B. The user equipment terminal may periodically or in response to a particular event send CQI reports to the respective serving e-Node B, which indicate the recommended transmission format for the next transmission time interval (TTI). The report may be constructed in such a way that it indicates the expected supported transport block size under certain assumptions, which may include, the recommended number of physical resource blocks (PRB), the supported modulation and coding scheme, the recommended multiple input multiple output (MIMO) configuration, as well as a possible power offset.
In general, the interface between a user equipment (UE) and the UTRAN or E-UTRAN has been realized through a radio interface protocol established in accordance with radio access network specifications describing a physical layer (L1), a data link layer (L2) and a network layer (L3). For example, the physical layer (PHY) provides information transfer service to a higher layer and is linked via transport channels to a medium access control (MAC) layer of the second layer (L2). Data travels between the MAC layer at L2 and the physical layer at L1, via a transport channel. The transport channel is divided into a dedicated transport channel and a common transport channel depending on whether a channel is shared. Also, data transmission is performed through a physical channel between different physical layers, namely, between physical layers of a sending side (transmitter) and a receiving side (receiver).
Typically, the second layer (L2) may include the MAC layer, a radio link control (RLC) layer, a broadcast/multicast control (BMC) layer, and a packet data convergence protocol (PDCP) layer. The MAC layer maps various logical channels to various transport channels. The MAC layer also multiplexes logical channels by mapping several logical channels to one transport channel. The MAC layer is connected to an upper RLC layer via the logical channel. The logical channel can be divided into a control channel for transmitting control plane information, such as control signaling, and a traffic channel for transmitting user plane information, such as data information.
Due to the different capabilities of the eNodeB and UE, the downlink (DL) and uplink (UL) physical layers for LTE may be different. Physical channels convey information from higher layers in the LTE stack, and physical signals may be used exclusively for use in the physical layer. Physical channels map to transport channels, which are service access points (SAPs) for the L2/L3 layers. The downlink physical channels are Physical Downlink Shared Channel (PDSCH), which is used for data and multimedia transport, Physical Downlink Control Channel (PDCCH), which conveys UE-specific control information, and Common Control Physical Channel (CCPCH), which is used to carry cell-wide control information. There are two types of physical signals, reference signals used to determine the channel impluse response (CIR), and synchronization signals which convey network timing information. The downlink transport channels are Broadcast Channel (BCH), Downlink Shared Channel (DL-SCH), Paging Channel (PCH), and Multicast Channel (MCH).
In the uplink, the physical channels are Physical Uplink Shared Channel (PUSCH) and Physical Uplink Control Channel (PUCCH), which carry control information such as channel quality indication (CQI), ACK/NACK, HARQ and uplink scheduling requests. The uplink physical signals are uplink reference signal and random access preamble. The uplink transport channels are Uplink Shared Channel (UL SCH) and Randon Access Channel (RACH).
In order to facilitate the scheduling on the shared channel, the eNodeB transmits an allocation in a downlink shared control channel to the user equipment (UE). The allocation information will often be related to both uplink and downlink. In LTE for example, if the UE is scheduled for both uplink and downlink transmission the UE may receive two allocation grants, one for the uplink and one for the downlink. The functionality of the allocation is in principle similar to the high speed shared control channel (HS-SCCH), which is used for high speed downlink packet access (HSDPA).
The allocation is used to signal which user(s) are going to be scheduled in each TTI. The current default assumption in 3GPP is that the allocation includes information about which resource blocks in the frequency domain are allocated to scheduled user(s), which modulation scheme to use, what the transport block size is, and the like. The allocation also often includes various information related to hybrid automatic repeat requests (HARQ).
The current working assumption of an evolved UTRAN is that LTE will be using Orthogonal Frequency Division Multiplexing Access (OFDMA) as the multiplexing technique in the downlink direction, where multiple users can be frequency multiplexed in the downlink direction with a single TTI (which will have the duration of two sub-frames, i.e., 1 ms). One of the key elements for efficient link operation is the utilization of HARQ, such that for each transmitted packet, physical resources will be allocated in the uplink, so that each allocated UE can transmit HARQ acknowledgement or negative acknowledgement (ACK/NACK) based on its reception. The assumption for the downlink is that HARQ is asynchronous, but it is expected that the UE's transmission of ACK and NACK will be time-wise tied to the received transmission. In cases where UE does not have data to transmit in the uplink at the time of ACK/NACK, a dedicated physical control channel is assumed to carry the ACK/NACK bit. Otherwise, the ACK/NACK could also be piggy-backed to the data transmission. Both the allocation in uplink and downlink is decided and controlled by the eNodeB.
The number of users multiplexed in downlink may change significantly from sub-frame to sub-frame. Some of the factors contributing to such variations include changes in traffic (burstiness) which means that varying number of users have different and fast varying amounts of data to transmit, or properties of radio-aware scheduling that have changes in number of users allocated per sub-frame.
Given that the traffic is asymmetrical (or time-alternating as in the case of VoIP (Voice over Internet Protocol), it will often happen that ACK/NACK need be sent in uplink as data-non-associated transmission, or transmission on a separate physical channel tied to the downlink allocation, for example. Such resources need be reserved and provided if no adaptive mechanism is available, and we need to allocate the resources according to the worst-case multiplexing amount. This causes a loss in system capacity. Therefore, there is a need to overcome the problems discussed above.